This invention was made with State of California support under California Energy Commission Contract Number 500-97-031. The Energy Commission has certain rights to this invention.
The present invention relates to control of wind turbine blade pitches for parking conditions and in particular controlling pitches such that at least one of the blades is pitched for parking, such as with a pitch substantially differently from another blade.
An important factor affecting the cost of wind turbine construction and operation is the load force that must be borne by the wind turbine apparatus and various components thereof. In general; loads are related to wind velocity and, to ensure the desired structural lifetime of a wind turbine and its components, as well as to assure safety, the wind turbine structure and components must be designed to withstand the anticipated load (typically, plus a margin of safety).
In many wind turbine systems, the wind turbine is placed in a parked condition (wherein energy is not being generated) during conditions of high wind speed and/or turbulence, although parked configurations may be instituted under other conditions as well. In a typical parked condition, the rotor or blades are braked so as to avoid rotation.
Because the parked condition is often implemented during high wind conditions, when relatively high loads are generated, the wind turbine structures and components must be designed to withstand anticipated loads (plus a safety margin) generated during parked high wind condition situations. Indeed, for some designs, the high wind parked condition defines the highest anticipated loads on the wind turbine or components thereof and thus defines the structure (and, typically the cost) of the wind turbine and/or its components. Accordingly, it would be useful to provide a method and apparatus which can achieve a reduction in forces or loads on a wind turbine or its components, particularly during high wind, turbulent wind and/or parked conditions.
In at least some wind turbine configurations, the blades or rotor (as well as typically the generator and nacelle) can undergo at least some amount of yaw (i.e., rotation about a substantially vertically axis). High wind and/or turbulent conditions can result in a magnitude or rate of yaw which is undesirably large; such as creating undesirably large loads on a yaw bearing and the like). Accordingly, it would be useful to provide an apparatus and method which can reduce the magnitude or frequency of yaw, e.g., during high wind or turbulent wind events.
According to one aspect of the invention, during, or in anticipation of, high wind, turbulent wind and/or parked conditions or configurations, at least one blade is positioned with a pitch so as to act substantially as a rudder, preferably having a pitch substantially different from at least one other blade of the turbine. Preferably, the two pitches are such that one blade is substantially more sensitive to (generates substantially larger yaw torque in response to) changes in wind direction than at least one other blade. In one embodiment, one of the blades is provided with a substantially 0xc2x0 effective pitch (such that the effective or average airfoil axis or cross-sectional axis is substantially vertical) while at least one other blade is substantially feathered, having approximately a 90xc2x0 pitch (such that the effective or average airfoil or cross-sectional axis is substantially horizontal). In such a configuration, the zero-pitch blade acts generally as a rudder, causing the rotor to yaw so that all blades are positioned with free ends in a plane substantially parallel to the wind direction, with the free end of the zero-pitch blade pointing downstream.
Preferably, the wind turbine is provided with actuators and controls capable of achieving the described pitch, such as pitch actuators which can set pitches of two or more blades to different values and/or computer-implemented or other procedures for generating the described pitch and/or calculating desired or preferable pitches.
In one embodiment, the present invention involves providing a wind turbine which can be placed in such load-and/or yaw-reducing configuration and which has reduced cost, (such as reduced structural, tower and/or bearing costs) such as resulting from reduced anticipated loads and/or yaw, e.g., compared with the cost that would be required without the load and/or yaw reduction achieved according to the present invention.
According to one aspect, a wind turbine is configurable with at least two different blades having two different blade pitches. In one aspect, at least one blade is pitched to provide relatively high sensitivity to wind direction or sheer so as to perform a rudder function such that the vertical plane defined by the blade ends is substantially parallel to the wind direction. In one embodiment, the rudder action configures a two bladed wind turbine such that the non-rudder blade, having a pitch different from the rudder blade, has its free end pointing substantially directly into the wind, providing relatively low cross-section for resistance to the wind and accordingly relatively low load.